1. Field of the Invention
The present invention relates to a radiation generating tube which includes a transmission target. The present invention relates also to radiation generating apparatus in which the radiation generating tube is used.
2. Description of the Related Art
A transmission radiation generating tube is a vacuum tube including a cathode, an anode and an insulating tubular side wall. Electrons emitted from an electron source of the cathode are accelerated by high voltage applied between the cathode and the anode. The electrons collide with a transmission target on the anode and cause radiation to generate. The emitted radiation is extracted outside through a transmission target. The transmission target also functions as a radiation extraction window. Such a transmission radiation generating tube is used in radiation generating apparatus for medical and industrial use.
Such a transmission radiation generating tube and a reflective radiation generating tube have had a problem about how to improve their voltage withstanding capability. Japanese Patent Laid-Open No. 9-180660 describes a technique to improve voltage withstanding capability. In the described transmission radiation generating tube, a cathode-side end of an electron-focusing electrode is disposed between a tubular side wall and a cathode and is fixed thereto. A gap is formed between the tubular side wall and the focusing electrode. Since creepage distance of the tubular side wall is thus elongated, voltage withstanding capability is improved. Japanese Patent Laid-Open No. 2010-086861 and “Development of Portable X-ray Sources Using Carbon Nanostructures—A step toward X-ray nondestructive inspection and Rontgen examination using dry batteries as a power source” (Translation of AIST press release of Mar. 19, 2009) {http://www.aist.go.jp/aist_e/latest_research/2009/20090424/20090424.html}each describe a technique to improve voltage withstanding capability by providing an intermediate potential electrode (“intermediate electrode”) in a reflective radiation generating tube.
If, however, further improvement in voltage withstanding capability is desired in these techniques described above, the following problems may arise. In the technique described in Japanese Patent Laid-Open No. 9-180660, local potential of the tubular side wall is determined in accordance with a dielectric constant (or volume resistivity in certain cases) of the tubular side wall. There is, therefore, a possibility that electrical discharge occurs between the focusing electrode and an inner wall of the tubular side wall in some situations depending on the distance from the focusing electrode and from the inner wall of the tubular side wall. In the techniques described in Japanese Patent Laid-Open No. 2010-086861 and “Development of Portable X-ray Sources Using Carbon Nanostructures—A step toward X-ray nondestructive inspection and Rontgen examination using dry batteries as a power source”, since the intermediate electrode protrudes further toward an inner space than an inner wall surface of the tubular side wall, electrons are emitted at an end portion of the intermediate electrode or from between a boundary of the intermediate electrode and the inner wall of the radiation generating tube. There is, therefore, a possibility that electrical discharge occurs between the intermediate electrode and the anode.
It occurred to the present inventors to suitably define the potential of the intermediate electrode in order to reduce the electrical discharge. However, there is still a possibility that electrical discharge occurs between the intermediate electrode and the focusing electrode or between the intermediate electrode and the electron source even in a structure in which the potential of the intermediate electrode is suitably defined. If electrical discharge occurs, the potential of the intermediate electrode may be lowered quickly. In some cases, depending on an electrification state of the tubular side wall, secondary electrical discharge may be caused between the anode and the focusing electrode, or between the anode and the cathode.